The 15-million-year-old marine sediments exposed at Calvert Cliffs on the Western Shore of the Chesapeake Bay are famous for giving up fossil shark teeth. But until now, scientists could only speculate about what the sharks were biting.
Now they've gotten a glimpse - and it's not a pretty picture.
In a paper published this month in the German science journal Naturwissenschaften, paleontologists reported that the Calvert sands have coughed up a pair of fossilized feces. Both bear the bite marks of prehistoric sharks.
The poop, perhaps from a marine crocodile, is believed to be the first described in the scientific literature that preserves the tooth marks of a vertebrate animal. Such finds suggest behaviors scientists could never deduce from fossil bones and teeth alone, say the paper's authors, Stephen J. Godfrey, curator of paleontology at the Calvert Marine Museum in Solomons, and his former assistant, Joshua B. Smith.
"It's an amazing little story," Godfrey said. "It's got the hook, and then you can feed people the science, a paleontological version of CSI."
Coprolites, or fossils of animal dung, are not common along the Calvert Cliffs, but they do turn up from time to time, he said. Four examples found earlier hold preserved remnants of feathers, suggesting crocs rising to gulp down swimming gannets or shearwaters.
The shark-bitten coprolites were found on the beach by W. "Douggie" Douglass, a local collector. He took the first sample to the museum about six years ago, and the second two years later, Godfrey said. One of them, measuring 4 1/2 inches long, bears tooth marks on both sides. The other, 5 1/4 inches long, appears to have been bitten off from a larger piece, leaving tooth impressions along one edge.
Casts made from the tooth marks led Godfrey and Smith to conclude the biter was probably an extinct species of tiger shark, perhaps 10 feet long.
But really ... what were those crazy fish up to?
It might have been "aborted coprophagy," the scientists said, polite terminology that a Calvert Marine Museum news release translated as: "[T]he prehistoric sharks started to eat the turds, but then thought better of it."
Modern sharks are not known to eat such things on purpose, the authors write. But they do engage in what the paper calls "benthic or nektonic exploration." That is, they swim around, sampling tasty-looking things lying on the bottom, or floating in the water. They spat these out.
The authors' third guess is predation, and it's no less icky: "The sharks bit into the feces-filled abdomen or dismembered intestines of a prey animal (alive or dead), leaving tooth impressions in the feces that subsequently sank to the bottom of the ocean and fossilized."
Fourteen million years later, the coprolites have finally delivered their message: Maryland's Miocene ocean was a nasty place.